1. Field of the Invention
This invention relates generally to methods and compositions for treating Bayer process streams. More particularly, it relates to solvent extraction methods that utilize an organic salt to remove undesired constituents (such as oxalate) from Bayer process streams.
2. Description of the Related Art
The almost universally used process for the manufacture of alumina is the Bayer Process. In a typical commercial Bayer Process, raw bauxite is pulverized to a finely divided state. The pulverized ore is then fed to a slurry mixer where a slurry is prepared using spent liquor and added caustic. This bauxite slurry is then diluted and sent through a series of digesters where, at about 300°-800° F. and 100-2000 p.s.i., about 98% of the total available alumina is extracted from the ore which may contain both trihydrate and monohydrate forms of alumina. The effluent from the digesters passes through a series of flash or blow-off tanks wherein heat and condensate are recovered as the digested slurry is cooled to about 230° F. and brought to atmospheric pressure. The aluminate liquor leaving the flashing operation typically contains about 1-20% solids, which include the insoluble residues that remain after reaction between the bauxite ore and basic material used to digest the ore and the insoluble components which precipitate during digestion.
The coarser solid particles are generally removed with a “sand trap” cyclone. To separate the finer solid particles from the liquor, the slurry is typically fed to the center well of a mud settler (also called a decanter, a residue thickener or a raking thickener) where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution, referred to as “green” or “pregnant” liquor, overflows a weir at the top of the mud settling tank and is passed to subsequent processing steps. The settled solids (“red mud”) are withdrawn from the bottom of the mud settler and passed through a countercurrent washing circuit (called “the washer train”) for further recovery of sodium aluminate and soda. Aluminate liquor overflowing the settler (settler or thickener overflow) still contains various impurities, both dissolved and undissolved, including typically 50 to 200 mg of undissolved suspended solids per liter. This liquor is then generally further clarified by filtration to remove undissolved suspended solids to give a filtrate with about 10 mg or less of undissolved suspended solids per liter of liquor. Alumina, in relatively pure form, is then precipitated from the filtrate as alumina trihydrate crystals. The remaining liquid phase or spent liquor may be concentrated to form “strong” liquor, from which additional alumina trihydrate may be precipitated and from which additional spent liquor may be generated. The spent liquor streams are typically returned to the initial digestion step and employed as a digestant of additional ore after being reconstituted with additional caustic.
Bauxite ore generally contains organic and inorganic impurities, the amounts of which are specific to the bauxite source. During the early stages of digestion, Bayer liquor contains a wide variety of organic compounds including polybasic acids, polyhydroxy acids, alcohols and phenols, benzenecarboxylic acid, humic and fulvic acids, lignin, cellulose, and other carbohydrates. Under alkaline, oxidative conditions such as those existing in the Bayer system these complex organic molecules break-down to form other compounds such as sodium salts of formic, succinic, acetic, lactic and oxalic acids. Predominant among these is sodium oxalate.
Sodium oxalate has a low solubility in caustic solutions and thus, if not adequately controlled, tends to precipitate in an acicular (fine, needle-like) form in regions of the Bayer circuit where there is an increase in causticity or decrease in temperature. These fine sodium oxalate needles can nucleate alumina trihydrate and inhibit its agglomeration, resulting in fine, undesirable gibbsite particles which are difficult to classify and are less than ideal for calcination. The excessive generation of fine particles can lead to blocking of the pores in the filter cloths during filtration of the thickener overflow liquor, hence undesirably decreasing the rate of filtration.
During the calcination stage, oxalate can decompose to leave fragile alumina particles having high sodium content, which in turn can increase the cost of aluminum production and subsequently produce undesirable levels of CO2 emissions. Additionally, due to the formation of sodium oxalate: (1) scale growth may be increased; (2) there may be an increase in liquor boiling point; (3) caustic losses may be observed in the circuit (due to the formation of organic sodium salts); and/or (4) the Bayer liquor viscosity and density may be increased, resulting in increased material transport costs.
The presence of oxalate and/or other organic species such as glucoisosaccharinate, gluconate, tartrate, and mannitol may decrease gibbsite precipitation yield. The presence of gluconate may reduce gibbsite growth rate. The presence of humic substances in Bayer liquor is common. Due to their surfactant nature, medium and high molecular weight humic substances are often responsible for liquor foaming and interference with red mud flocculation. High levels of organic material in Bayer liquor may also result in a decrease in coagulation efficiency and supernatant clarity during the red mud circuit. Alumina trihydrate containing high levels of organic matter also tends to produces a final product having an undesirably high level of coloration and/or impurity level.
As the Bayer process is cyclic, organic matter entering the process stream tends to accumulate with each cycle of the process, with steady state impurity concentration determined by process input and output streams. The major organic exits are the red mud circuit with the gibbsite product, via oxidation to carbon dioxide or carbonate and via any organic removal steps in place.
Methods of dealing with the organic impurity problem have been discussed. See, e.g.,: Foster and Roberson, Light Met., (1988), 79; U.S. Pat. No. 7,067,106; Tran et al., Light Met., (1986), 217; Stuart, Light Met., (1988), 95; Yamada et al., Light Met., (1981), 117; Brown, Light Met., (1989), 121; U.S. Pat. No. 4,280,987; Shibue et al., Light Met., (1990), 35; Kumar, Light Met., (1991), 1229; Hollanders and Boom, Light Met., (1994), 91; Perrotta and Williams, Light Met., (1995), 77; Perrotta and Williams, Light Met., (1996), 17; Williams and Perrotta, Light Met., (1998), 81; U.S. Pat. No. 4,496,524; The and Bush, Light Met, (1987), 5; Pulpeiro et al., Light Met., (1998), 89; Farquharson et al., Light Met., (1995), 95; U.S. Pat. No. 5,385,586; U.S. Pat. No. 4,036,931; Bangun and Adesina, App. Catalysis A: Gen., (1998), 175:221; Pareek et al., Adv. Environ. Res., (2003), 7:411; WO 97/22556; Atkins, and Grocott, Light Met., (1993), 151; Cousineau and The, Light Met., (1991), 139; U.S. Pat. No. 4,902,425; U.S. Pat. No. 5,284,634; WO 07/066143. However, despite these efforts, a long-felt need exists for improved methods of removing impurities from Bayer process streams.